We could see some confrontations around this country tomorrow. The group ACT for America is known for its criticism of Islam, and it's leading a march against Sharia in at least 19 states. Sharia laws are principles that guide the behavior of observant Muslims. Some civil rights leaders see these marches as provocative, and counterprotests are likely, as NPR's Tom Gjelten reports.

TOM GJELTEN, BYLINE: These marches are largely the brainchild of Scott Presler, a 29-year-old Republican activist now working with ACT for America.

SCOTT PRESLER: This was born on the Internet from me sending out a Twitter post saying, hey, I would love to do a rally. Who wants to help?

GJELTEN: As a gay man, Presler says he was first motivated to fight Muslim extremism after the deadly Orlando shooting last June. The target there was a gay nightclub. The perpetrator allegedly pledged loyalty to ISIS. When he proposed the idea of an anti-Sharia rally, he says he got a flood of responses.

PRESLER: My email has been just pummeled with activity. We have people who still want to have their own rallies across the country.

DENISE ZAMORA: So this is going to be our meeting spot. We're going to do a prayer. I have a singer that's going to sing the national anthem and "My Country, 'Tis Of Thee."

GJELTEN: Denise Zamora is one of those who responded to Scott Presler's call. She's organized a march against Sharia in San Bernardino, Calif. It'll be near the site of the 2015 shooting there, when a radicalized Pakistani-American and his wife shot and killed 14 people.

ZAMORA: I wasn't even familiar with anything about Sharia law. I didn't really know much about radical Muslims, jihad. I wasn't educated. So I started learning a little bit about, you know, everything. And when ACT for America announced that they were going to do a nationwide march, I quickly volunteered.

GJELTEN: Zamora's big issue and one highlighted in all the marches is female genital mutilation practiced in some conservative Muslim societies. But the marches are in danger of veering in an extremist direction. ACT for America has a somewhat militant attitude toward Islam, and the marches are now attracting a large number of far-right sympathizers. In Arkansas, one was organized by Billy Roper, a white supremacist who touted the marches this week on his podcast.

(SOUNDBITE OF ARCHIVED RECORDING)

BILLY ROPER: We want to send a message to Muslims that they are not welcome in our communities. They're not welcome in our state. And ultimately, want to send a message to Muslims that they are not welcome in our nation and, of course, endgame, on our planet.

GJELTEN: Roper specifically invited white nationalists to attend the marches and bring weapons. When word of the podcast got out Scott Presler, the national organizer, put out a statement that the Arkansas march was no longer sanctioned by ACT for America. But left-wing militants had already promised to show up at the anti-Sharia rallies and confront the marchers. Right-wing paramilitary groups such as the Oath Keepers offered to provide private security at the rallies.

STEWART RHODES: We're encouraging our members to go to each and every one of them.

GJELTEN: Stewart Rhodes is the Oath Keepers president. Rhodes says the Oath Keepers will defend the anti-Sharia speakers and marchers from anyone who challenges them.

RHODES: You know, where the police stand up, we'll stand down. But where they stand down, we have to stand up.

GJELTEN: And that's the scenario where trouble could ensue given that Saturday's marches are now due to take place in at least 27 cities across the country. Tom Gjelten, NPR News.